This invention relates to a decoder circuit for generating logical signals in response to the actuation of a keyboard provided with symbols associated with respective logical signals for use with a keyboard which is provided with a plurality of symbol fields. The symbol fields are adapted to be provided with respective ones of said symbols and are separated by gaps between adjacent keys and defined by a plurality of adjacent keys. The adjacent keys are adapted to be jointly depressed by a finger of an operator, wherein a plurality of adjacent keys are associated with each of said symbol fields in such a manner that each key of the keyboard, with the exception of keys disposed at the edges of the keyboard, is associated with four adjacent symbol fields. Only alternate keys of each column of keys of the keyboard are connected to electric switches for generating the logical signals. Each of these switches comprises a contact connected to one of a plurality of row conductors associated with respective rows of keys; and another contact connected to one of a plurality of column conductors associated with respective columns of keys.
Decoder circuits may be used in association with keyboards which are of the type briefly described hereinbefore and disclosed in, for example, German Pat. specification No. 27 29 157 corresponding to U.S. Pat. No. 4,400,593, assigned to the assignee hereof.
In the prior art keyboard described in U.S. Pat. No. 4,400,593, the number of keys of each row of keys is larger by one than the number of symbols associated with each row; and the number of keys of each column of keys is larger by one than the number of symbols associated with the said column. For this reason, the number of row conductors connected to respective rows of keys is larger by one than the number of symbols of a row of symbols; and the number of column conductors associated with a column of keys is larger by one than the number of symbols of a column of symbols.
If the signals that can be derived from the row conductors and column conductors are decoded in such a manner that input signals are delivered to individual column conductors and the output signals are then read from the row conductors, the above-mentioned difference between the number of symbols in a row of keys and the number of row conductors, and between the number of symbols in a column of keys and the number of column conductors, will give rise to difficulties if such decoding is effected with known decoder circuits.